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Investigation of the major tectonic structures in the SW Baltic sea by means of deep reflection seismic studies : a major element of BABEL

Objective

To investigate the deep structure and evolution of the transition from Precambrian to younger tectonic domains of the continental crust by applying the marine reflection seismic method to major tectonic features (such as Tornquist Line, Ringkoebing-Fyn High) in the southern Baltic Sea. These features have all been associated with evolution of continental crust and the development of sedimentary basins (and their thermal history which affects the nature of both hydrocarbon and geothermal resources).
Deep crustal reflection profiles and collinear wide angle/refraction profiles in the Baltic Sea and the Gulf of Bothnia have been obtained. The structure of the Precambrian Baltic shield and its transition to the Phanerozoic crust of western Europe across the Tornquist Zone has been studied.

Research has been carried out into the processes of continental crust formation and evolution in the Baltic Sea, with special emphasis on the development of sedimentary basins in various tectonic settings and ages. Seismic and electromagnetic studies were undertaken to provide detailed sections of the crust and upper mantle in areas that could lead to the identification of new and previously unsuspected geothermal reservoirs, hydrocarbon and mineral deposits. The field research showed a unique dense coverage of deep reflection lines recorded by the marine technique in an ancient shield area, with 68 land stations clustered intensely in the neighbourhood. The Baltic Shield is seen to contain many different provinces with distinct and sometimes strong reflectivity throughout the whole crust, which reaches depths up to 60 km. Major boundaries in the Proterozoic terrain were observed and the results of more than 1000 million years old magmatic processes imaged.

The BABEL campaign involved a dense coverage of deep reflection lines recorded by marine techniques in an ancient shield area and land stations clustered in the neighborhood of the marine survey recorded the seismic signals of airgun shots 50 m apart, resulting in a wide angle of refraction coverage with a density never achieved before. As a consequence, data quality was generally excellent, and the tremendous amount of collected data is not yet all processed or finally interpreted. Many correlations between geological observations in Sweden and the reflectivity picture of BABEL lines A and B have been established. Special attention was given to the south west rim of the Baltic shield where apparently 3 major fault systems herald the complex transition to the Caledonian and Variscan terranes in Denmark and north Germany.
The Tornquist Zone (TZ) presents itself as a complex faulted block structure of 50 km width. Some blocks have rotated, others show an inversion tectonics. In the middle crust, traces of fault zones are observed and the reflective, and probably ductile, lower crust shows a smooth upwelling below TZ and on its north east flank. The transition to the mantle is also anomalous, showing apparently 2 Moho discontinuities with a velocity around 7.4 km/s indicating a mixed or extremely mafic compostion at the bottom of crust. On the east flank signs of fault branches are found in the uppermost mantle and the wide angle studies reveal a strong north east dipping reflector at depths of 50 km to 70 km.
BABEL line A displays 3 distinctly different reflectivity provinces, separated by TZ and branches of the Caledonian deformation front (CDF): an extremely strong reflective diffractive part in the north east; a part with Caledonian nappes in the uppermost part and laminated lower crust in the middle section; and an irregular and weak reflective part in the south west, now believed to be north east boundary of the terrane Cadomia (East Avalonia). The variation of reflectivity in an old shield area is nearly as large as in the Phanerozoic.
The structure of the continental crust in the southern Baltic sea is investigated using the marine near-vertical reflection seismic method. The most modern seismic equipment available was used on board a commercial research vessel. Beside the prominent Tornquist Line and Ringkoebing-Fyn High other key structures were studied.

Additional refraction/wide-angle data were recorded by numerous land stations up to distances of 700 km using a tuned air gun array and a multifold recording pattern.

The data will be used for different approaches to a joint interpretation with the aim of :

1) resolving the fine structure of the crust and its variations across important tectonic lineaments;

2) resolving the velocity-depth distribution across these tectonic lineaments;

3) improving the understanding of the evolution of continental crustal provinces, their amalgamation along sutures and the interaction with major stresses.

Coordinator

Christian-Albrechts-Universität Kiel
Address
Olshausenstraße 40-60
24118 Kiel
Germany

Participants (5)

PRAKLA-SEISMOS GmbH
Germany
Address
Buchholzer Straße 100
30655 Hannover
ROYAL HOLLOWAY AND BEDFORD NEW COLLEGE
United Kingdom
Address
Egham Hill
TW20 0EX Egham, Surrey
THE CHANCELLOR, MASTERS AND SCHOLARS OF THE UNIVERSITY OF CAMBRIDGE
United Kingdom
Address
Trinity Lane, The Old Schools,
CB2 1TN Cambridge
UNIVERSITY OF AARHUS
Denmark
Address
1,Nordre Ringgade 1-3
8000 Aarhus C
UNIVERSITY OF COPENHAGEN
Denmark
Address
Noerregade 10
1017 Koebenhavn K/copenhaegen